1. Field of the Invention
The present invention relates to sintered silicon nitride and a silicon nitride tool, and more particularly, to sintered silicon nitride having high mechanical strength and resistance to thermal shock; and providing, even when employed to produce a high-speed milling cutter for working ductile cast iron, a cutter having a cutting edge which is not prone to chipping, thereby attaining excellent tool life, and to a silicon nitride tool formed of the sintered silicon nitride.
2. Description of the Related Art
Silicon nitride, having excellent resistance to chemicals (acids) and heat resistance to high-temperature molten metal, is employed to form crucibles and nozzles. In addition, silicon nitride is inert to the living body, and thus is employed as a biological material for producing, for example, prosthetic joints.
Since sintered silicon nitride, which is produced by sintering in the presence or absence of an additive, is an insulator, the sintered silicon nitride is employed as a substrate of a multi-layer substrate or as insulative material included in Si transistors. Sintered silicon nitride finds a variety of applications, including heater substrates, brake mechanisms for an elevator, and blades for gas turbines by virtue of its low thermal expansion coefficient and high thermal shock strength; and applications such as cutting tools and bearings by virtue of its high wear resistance.
Conventionally, silicon nitride tools have been employed as cutting tools for turning ordinary cast iron (FC; ferrous cast) materials, particularly for turning at comparatively low cutting rates of about 500 m/min.
In recent years, a decrease in weight of automobile parts generally made of FC material is envisioned in order to reduce automobile fuel consumption. Accordingly, there is an increasing demand for ductile cast iron (FCD; ferrous cast, ductile) materials which can provide, as compared with FC materials, thin and lightweight parts. Furthermore, in order to work such parts at low cost and high efficiency, cutting must be performed at high speed.
Sintered silicon nitride is known to have high mechanical strength and toughness and is employed in a tool for cutting FC materials. In this regard, Japanese Patent Application Laid-Open (Kokai) No. 11-268957 discloses a silicon-nitride-based cutting tool said to have high mechanical strength and toughness, a cutting edge which does not chip during cutting (turning) of FC materials, and a long tool life.
3. Problems to be Solved by the Invention
However, when FCD material (having mechanical strength higher than that of FC material) is cut with a sintered silicon nitride tool, the mechanical strength and fracture toughness thereof is insufficient, thereby causing chipping of cutting edges. Thus, there has been a problem that, when used to cut FCD material, the cutting edges of the silicon nitride cutting tool disclosed in the aforementioned Japanese Patent Application Laid-Open (kokai) No. 11-268957 become chipped, resulting in a problematically short tool life.
Chipping of a cutting edge is also induced by thermal cracking caused by concentration of heat generated in the cutting edge during high-speed working. In order to prevent concentration of heat generated during working and induced thermal cracking, the thermal conductivity of silicon nitride must be increased. Even though thermal cracking occurs, chipping of the cutting edge of the tool can be prevented so long as propagation of cracking can be prevented. In other words, by increasing fracture toughness of silicon nitride at an initial stage of crack propagation, chipping of the tool can be prevented, thereby attaining a long tool life, even though thermal cracking occurs.
In this regard, Japanese Patent Application Laid-Open (kokai) No. 2001-10864 discloses a silicon nitride material of enhanced thermal conductivity produced by reducing the amounts of aluminum and oxygen which are prone to form a solid solution in silicon nitride grains. Japanese Patent Application Laid-Open (Kokai) No. 2001-19556 discloses a silicon nitride material of enhanced thermal conductivity produced by controlling the ratio SiO2/Re2O3 (Re: rare earth element) in the grain boundary phase and by crystallizing the grain boundary phase.
However, when thermal conductivity above is enhanced, propagation of thermal cracking caused by thermal expansion and shrinkage of silicon nitride during high-speed milling involving repeated thermal shock is not completely suppressed. Thus, problematically, a satisfactory effect for preventing the propagation of thermal cracking can be attained only when the fracture toughness at an initial stage of propagation of cracking is enhanced together with thermal conductivity.